1. Field of the Invention
The invention relates to an electric power steering apparatus.
2. Description of the Related Art
In an electric power steering apparatus, if an alternating-current motor for steering assist cannot be driven due to an abnormality, the alternating-current motor is rotated in accordance with a steering operation performed on a steering wheel. In this case, the alternating-current motor serves as an electric power generator, and, for example, a battery of a vehicle is charged with the electric power generated by the alternating-current motor. In this case, the steering resistance includes, in addition to the friction resistance at a movable portion, the resistance for generating electric power by converting kinetic energy to electric energy (hereinafter, referred to as “power generation resistance”). In order to reduce the steering resistance, an electric power steering apparatus, in which multiple feed lines that connect a motor drive control circuit to an alternating-current motor are provided with switches, has been developed. In this electric power steering apparatus, if an abnormality occurs, the switches are turned off to disconnect the motor drive control circuit and the alternating-current motor from each other. Thus, the power generation resistance is excluded from the steering resistance. As a result, it is possible to reduce the steering resistance at the time of occurrence of an abnormality (refer to, for example, Japanese Patent Application Publication No. 2007-306720 (JP-A-2007-306720).
In the above-described electric power steering apparatus, the connection among phase coils of the alternating-current motor is the so-called star-connection (Y-connection). Therefore, if all the switches provided in the feed lines are turned off, a closed circuit, which includes the phase coils of the alternating-current motor and through which electric currents flow, is no longer present. As a result, it is possible to reduce the steering resistance. However, when the connection among the phase coils of the alternating-current motor is the ring-connection (delta-connection), even if all the switches provided in the feed lines are turned off, the phase coils, connected to each other in the ring-connection manner, still form a closed circuit and circulation currents flow through the closed circuit. Therefore, it is not possible to sufficiently reduce the steering resistance at the time of occurrence of an abnormality.